﻿<!DOCTYPE html>

<html lang="en" xmlns="http://www.w3.org/1999/xhtml">
<head>
    <meta charset="utf-8" />
    <title></title>
</head>
<body>
<script>
    //30行 
    //core_strundefined = typeof undefined, 
    //xml下判断变量为undefined可能证据不足，所以采用字符串'undefined'的判断方式
    alert(window.a == undefined);
    alert(typeof window.a == 'undefined');

</script>
</body>
</html>